The present invention relates generally to an anode assembly for use within x-ray generating devices and x-ray systems and, more specifically, to x-ray tubes and x-ray systems having a rotating anode assembly.
Typically, an x-ray generating device, referred to as an x-ray tube, includes opposed electrodes enclosed within a cylindrical vacuum vessel. The vacuum vessel is commonly fabricated from glass or metal, such as stainless steel, copper, or a copper alloy. The electrodes include a cathode assembly positioned at some distance from the target track of a rapidly rotating, disc-shaped anode assembly. Alternatively, such as in industrial applications, the anode assembly may be stationary. The target track, or impact zone, of the anode assembly is generally fabricated from a low expansivity refractory metal with a high atomic number, such as a molybdenum or tungsten alloy. Further, to accelerate electrons used to generate x-rays, a voltage difference of about 60 kV to about 140 kV is commonly maintained between the cathode and anode assemblies. The hot cathode filament emits thermal electrons that are accelerated across the potential difference, impacting the target zone of the anode assembly at high velocity. A small fraction, typically less than 1%, of the kinetic energy of the electrons is converted to high-energy electromagnetic radiation, or x-rays, while the balance is contained in back-scattered electrons or converted to heat. The x-rays are emitted in all directions, emanating from a focal spot, and may be directed out of the vacuum vessel along a focal alignment path. In an x-ray tube having a metal vacuum vessel, for example, an x-ray transmissive window is fabricated into the vacuum vessel to allow an x-ray beam to exit at a desired location. After exiting the vacuum vessel, the x-rays are directed along the focal alignment path to penetrate an object, such as a human anatomical part for medical examination and diagnostic purposes. The x-rays transmitted through the object are intercepted by a detector or film, and an image of the internal anatomy of the object is formed. Likewise, industrial x-ray tubes may be used, for example, to inspect metal parts for cracks or to inspect the contents of luggage at an airport.
Since the production of x-rays in a medical diagnostic x-ray tube is by its very nature an inefficient process, the x-ray tube components operate at elevated temperatures. For example, the temperature of the anode""s focal spot may run as high as about 2,700 degrees C., while the temperature in other parts of the anode may run as high as about 1,800 degrees C. The thermal energy generated during tube operation is typically transferred from the anode, and other components, to the vacuum vessel. The vacuum vessel, in turn, is generally enclosed in a casing filled with a circulating cooling fluid, such as dielectric oil, that removes the thermal energy from the x-ray tube. Alternatively, such as in mammography applications, the vacuum vessel may be cooled directly by air.
As discussed above, the primary electron beam generated by the cathode of an x-ray tube deposits a large heat load in the anode target. In fact, the target glows red-hot in operation. This thermal energy from the hot target is conducted and radiated to other components within the vacuum vessel. The oil or air circulating around the exterior of the vacuum vessel transfers some of this thermal energy out of the system. However, the high temperatures caused by this thermal energy subject the x-ray tube components to high thermal stresses that are problematic in the operation and reliability of the x-ray tube. As a result, x-ray tube components must be made of materials capable of withstanding elevated temperatures for extended periods of time. In particular, the anode assembly typically includes a shaft that is rotatably supported by a bearing assembly. The shaft may be made of, for example, high hardness tool steel. Due to the high temperatures associated with the operating target, the shaft may not be attached directly to the target. The integrity of the mechanical joint between the target and the shaft, however, must be maintained throughout service, since yielding at the mating surfaces may ultimately result in rotor-dynamic problems and possible premature x-ray tube failure.
Likewise, rotor-dynamic problems may be caused by unbalanced loading of the bearings. An x-ray tube bearing assembly generally consists of several sets of bearing balls and bearing races. A typical x-ray tube anode, and, specifically, a typical anode target, is configured such that the loads on the bearings proximal to the target are greater than the loads on the bearings distal to the target. This may lead to uneven and excessive wear on the bearing balls and races, especially in the presence of elevated temperatures. Uneven and excessive wear of the bearing balls and bearing races may result in increased friction, increased noise, and the ultimate failure of the bearing assembly.
The present invention overcomes the aforementioned problems and provides a rotor-dynamically stable, or rotationally balanced, anode assembly having bearings that are subjected to balanced loading conditions. This anode assembly takes advantage of gyroscopic effects, allowing it to rotate at high speeds. Further, the anode assembly of the present invention has a high integrity mechanical joint between the target and the shaft.
In one embodiment, an anode assembly for use within an x-ray tube includes a shaft having a first end and a second end, a target connected to the first end of the shaft, a rotor connected to the second end of the shaft, and spaced-apart bearings rotatably supporting the shaft, the shaft, target, and rotor positioned relative to each other such that the center of gravity of the combination of the shaft, the target, and the rotor is positioned between the bearings.
In another embodiment, an x-ray system includes a vacuum vessel having an inner surface forming a vacuum chamber; a shaft having a first end and a second end, the shaft supported by the vacuum vessel; an anode assembly disposed within the vacuum chamber, the anode assembly including a target connected to the first end of the shaft; a cathode assembly disposed within the vacuum chamber at a distance from the anode assembly, the cathode assembly configured to emit electrons that strike the target of the anode assembly and produce x-rays; a rotor connected to the second end of the shaft; and spaced-apart bearings rotatably supporting the shaft, wherein the shaft, target, and rotor are positioned relative to each other such that the center of gravity of the combination of the shaft, the target, and the rotor is positioned between the bearings.